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can be somewhat smoother than a simple 20% jump be-
tween the TBZB4 and TBZB5 samples. A micro-Raman
study will be undertaken and should help answer these
remaining questions.
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V. CONCLUSION
The results of the EXAFS analysis in the TBZB
samples points to Ti in a fourfold coordination as a net-
work former. Beyond this average view, the analysis of
the XAS spectra in the XANES range shows for the
sample with the highest TiO2-content the presence of a
small part of sixfold coordinated titanium, associated to
precrystalline nuclei. A quantitative evaluation of the ra-
tio of titanium atoms involved in these nuclei is obtained.
This evaluation matches with the optical measurements
of the refractive index in the glass system and explains
quantitatively the decrease of this index above a limit
value of TiO2 prior to any observable crystallization.
ACKNOWLEDGMENTS
The authors thank V. Mastelaro for providing the ref-
erence samples. They are greatly indebted to the staff of
LNLS for operating the storage ring and to its skillful
assistance of during XAS experiments. All beam time
was performed on an XAS beam line supported by LNLS
under the proposal XAS 69/97. This work was also par-
tially supported by FAPESP (PROC. No. 95/06439-4 and
99/02688-0).
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J. Mater. Res., Vol. 15, No. 3, Mar 2000
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